This invention relates to a tool for enlarging the ends of pipes which includes a toolholder body containing an axially movable mandrel having a tapering end and capable of being displaced by an externally applied force, and a toolhead exchangeably attached to the toolholder body co-axially with the mandrel and containing a plurality of expanding members in the form of cylinder segments radially displaceable by axial displacement of the mandrel and so mounted that they project axially from the toolhead, the segment edges facing the mandrel forming a recess corresponding to the mandrel taper.
Tools of this type are known (Published German Application No. 1,752,461). In this tool, the axial displacement of the mandrel inside the toolholder body is effected by a cam which cooperates with the rear face of the mandrel at the opposite end to that forming the elongated tapering end. The cam is operable by a lever arm carrying a handle or grip. A second arm carrying a handle is fixed to the body of the tool. This arrangement functions like a pair of caliper tongs from which the tapering end of the mandrel laterally projects. A toolhead containing the above defined expanding segments can be exchangeably screwed to the toolholder body. When the two arms are manually pressed together, they cause the mandrel to be displaced into the tapering recess in the center of the annulus of expanding segments which are thereby expanded and radially forced apart.. A pipe end fitted over the expanding segments will thus be enlarged sufficiently for it to receive the spigot end of another pipe. It will be apparent that, depending upon the material of which the pipes are made, considerable expanding forces must be generated.
Exchangeability of the toolhead is necessary to permit toolheads wherein the overall diameter of the annulus of expanding segments differs to be attached to the same toolholder body. However, the annulus of expanding segments must have a minimum overall diameter dictated by the major diameter of the mandrel. Otherwise the expanding segments would be too slim and lack the necessary mechanical strength to withstand the very high bending moments involved. It is therefore desired to keep the major diameter of the mandrel for a given angle of taper as small as possible in order to preserve the necessary mechanical strength. This has the undesirable result of impairing the functional reliability of the tool and of reducing the life of the expanding segments which is inherent in the manner in which the expanding segments are produced to endow them with the property of expandability.
The expanding segments are made from a solid cylindrical blank which is provided with an axial conical recess corresponding to the conically tapering end of the mandrel. An annular flange at one end of the cylinder serves for subsequently retaining the segments in the toolhead. However, for purposes of the present explanation this flange need not be taken into account. The cylinder containing the coned recess is then cut into say six equal segments. This is done with a sawblade which produces a specially wide cut because the material which the sawblade removes is intended to create the necessary gap that will enable the annulus of segments to be pushed radially closer together. In this contracted state the segments can then be inserted into the unexpanded end of a pipe for the purpose of being forced apart by the mandrel with the simultaneous enlargement of the pipe end. The segments are thus expanded until they attain the relative positions they originally occupied in the solid cylindrical blank from which they were cut. Fine castings produced to comply with these geometrical conditions that have been described would, of course, be equivalent.
However, the removal of material during the sawcut results in a significant reduction of the narrow surface areas that make contact with the mandrel. The sawcut moves the end of the conical recess from the bottom of the segments further up into the interior of the annulus of segments, and the mandrel which is forced into the interior of this annulus will not therefore make contact with a surface that extends from the top to the bottom end of each segment. This means that the specific contact pressure will be much higher, with a concomitant higher rate of wear and a tendency of the segments to tilt or of laterally slipping off the mandrel surface. Besides, a corresponding bending moment involving the unsupported projecting ends of the segments which are not backed by the tapering end of the mandrel will arise. While this may be acceptable when the expanding segments are fairly small, the problems become more significant as the diameter and length of the segments increase, assuming these segments are designed to cooperate with the same (small) mandrel. The expanding segments must necessarily define a greater diameter if they are to be used for wider gauge pipes wich usually also have a thicker wall, so that the magnitude of the radial expansion of the segments must also be correspondingly greater. This increased distance of radial separation necessitates the removal of more material during the subdivision of the cylindrical blank into the several segments and consequently the remaining surface available for contact with the tapering end of the mandrel will become unacceptably small.
The problem might be solved by using a toolholder body containing a larger mandrel, but this would preclude attaching the tool to a toolhead containing smaller expanding segments. For the great diversity of possible application, i.e., for pipes of a wide range of diameters, several expanding tools would therefore have to be kept available and the advantage afforded by the exchangeability of the toolhead containing the expanding segments would partly be lost.